In many cases, a spinning forming method that is one type of plasticity processing uses a mandrel (shaping die) attached to a main shaft for rotating a plate that is a formation target. For example, as disclosed in PTL 1, the plate is placed on the mandrel attached to the main shaft, and a tail stock presses a surface of the plate, the surface being opposite to a surface contacted by the mandrel. Thus, the plate is fixed to the mandrel. In this state, the plate is rotated by the rotation of the main shaft, and a processing tool, such as a processing roller, is brought into contact with the surface of the plate. Thus, the plate is formed.